Encapsulation method for maintaining biodecontamination activity

ABSTRACT

A method for maintaining the viability and subsequent activity of microorganisms utilized in a variety of environments to promote biodecontamination of surfaces. One application involves the decontamination of concrete surfaces. Encapsulation of microbial influenced degradation (MID) microorganisms has shown that MID activity is effectively maintained under passive conditions, that is, without manual addition of moisture or nutrients, for an extended period of time.

RELATED APPLICATION

This application claims priority from U.S. provisional application Ser. No. 60/142,351 filed Jun. 30, 1999 and is incorporated by reference.

CONTRACTUAL ORIGIN OF THE INVENTION

This invention was made with United States Government support under Contract No. DE-AC07-94ID13223, now Contract No. DE-AC07-99ID13727 awarded by the United States Department of Energy. The United States Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to decontamination of cementitious surfaces. More specifically the present invention is related to a microbiological process for removing radioactive contamination from a concrete surface.

2. Relevant Technology

In the nuclear industry, concrete is commonly used to retain radioactive substances. There are thousands of nuclear facilities comprising about 50 square miles of radionuclide-contaminated contaminated concrete surfaces. These nuclear facilities use concrete ponds, canals, sumps, and other structures for the containment, transport, and storage of liquid and solid radioactive materials, resulting in the contamination of the concrete surfaces with radionuclides. Typically, the contamination is securely fixed on, or within one or two millimeters of the surface. Before the concrete is disposed of in a suitable waste disposal site, it is desirable to decontaminate the radio-active concrete surface.

One conventional method for decontamination entails dismantling the entire structure. However, all of the rubble from a dismantled structure must be disposed of as radiological waste even though the contaminated portions may comprise only the outer few millimeters. Another option involves manually chipping off only the outer few millimeters of the contaminated concrete. By either method, the cost to decommission and decontaminate the contaminated sites is estimated to be tens of billions of dollars.

In response to the significant need for an alternative cost-effective method for decontaminating a cementitious surface, U.S. Pat. No. 5,414,196 by Jennings, the entire disclosure of which is hereby incorporated herein by reference, is directed to microbial degradation of contaminated cement surfaces. The method comprises applying a microorganism, such as a species of thiobaccillus, to degrade the surface of the concrete and thereby release a material comprising the radioactive substance. The released material is then removed from the surface by vacuum suction, scraping, brushing, or abrasion blasting. Once the concrete surface has been sufficiently decontaminated, the process may be stopped by heating or by depriving the microorganism of essential nutrients or moisture.

However, these same parameters (temperature, nutrients, and moisture) must be maintained at a level conducive to the growth of the bacteria in order for this method to perform effectively. For example, it is preferable that the cementitious surface contain a nutrient source such as sulphur. If the concrete does not contain sufficient nutrient, an alternate source of nutrient needs to be applied. Further, the needed moisture level must be maintained by periodic, manual addition of water.

Though such addition of moisture and nutrients is successful at maintaining active microbial influenced degradation of the concrete surfaces, manual addition is impracticable as the final method for maintaining the process under commercial application. Not only does this increase the cost and burden associated with decontaminating the various structures, it also increases the potential health and safety risks of the personnel responsible for the manual addition of moisture and nutrients.

SUMMARY AND OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide a method for maintaining microbial influenced degradation without the need for manual application of nutrients or moisture.

It is another object of the invention to provide a practical, cost effective, and environmentally acceptable method for decontaminating concrete.

A further object of the invention is to provide a method for decontaminating large surface concrete structures, hot cells, ponds, canals, sumps, biological shields, and other structures requiring decontamination through a naturally occurring microbial process.

These and other objects and features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.

To achieve the forgoing objects, and in accordance with the invention as embodied and broadly described herein, the present invention relates to a new and useful method for microbial influenced degradation of contaminated structures. The preferred method of the present invention is a biomediated process for removing contamination from a cementitious surface utilizing encapsulated microorganisms for the microbial influenced degradation. The encapsulated microorganisms of the present invention do not require the manual addition of moisture or nutrients to maintain conditions conducive to growth. Thus, the encapsulated microorganisms are simply sprayed or painted on to the contaminated surface and left for a period of time sufficient for microbial influenced degradation activity to degrade the cementitious surface to a selected depth.

The process preferably comprises applying a substantially uniform coating of the encapsulated microorganism to a contaminated cementitious surface. The microbial influenced degradation activity is preferably permitted to continue until the cementitious surface has been degraded to a desired depth. It has been demonstrated that radioactive contamination is usually localized in the first 1-2 mm of cementitious material. The degraded layer is preferably removed by brushing, washing or scrubbing the degraded cementitious surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Biodecontamination of concrete surfaces is aimed at decontaminating concrete contaminated by radioactive and other hazardous compounds. Microbial influenced degradation (MID) has been demonstrated as an effective means for decontaminating concrete surfaces. When implemented, this technology has the potential of greatly reducing the costs compared to existing technologies which rely on physical/chemical removal, and disposal of contaminated portions of buildings and structures.

Biodecontamination has been shown to be effective under controlled conditions in laboratory and field applications. With the use of sulfur oxidizing bacteria, for example, decontamination occurs through the biogenic production of sulfuric acid which subsequently removes the contaminated concrete surface. Studies have shown that the key conditions for promoting active MID are maintaining an available source of reduced sulfur and providing a moist near-surface environment. However, the needed moisture level and sulfur concentration must be maintained by periodic, manual addition of water and elemental sulfur. Though this method is successful in maintaining active MID of the concrete surfaces, manual addition of moisture and nutrients is impracticable for commercial application.

Therefore, the present invention is directed to a method for the maintenance of appropriate levels of moisture and nutrients at the concrete surface without the need for additional manual applications thereof. Specifically, the method of the present invention utilizes encapsulation of MID microorganisms to eliminate the need for manual application of moisture and nutrients for degradation of contaminated cementitious surfaces.

The necessary components of the present invention include a microorganism, an encapsulation material, and a substrate such as a contaminated cementitious surface. Each of these components will be described individually herein prior to discussion of the method of the present invention. The terms “a,” “an,” and “the” as used in the specification and the appended claims include plural referents unless the context clearly dictates otherwise. Thus, a reference to “a microorganism” includes a reference to two or more of such microorganisms, “a bacteria” includes two or more of such bacteria, etc.

The microorganisms utilized in accordance with the present invention preferably belong to group capable of performing microbial influenced degradation. MID microorganisms typically grow under environmental conditions with temperatures ranging from 0° C. to 30° C. All MID microorganisms require the presence of moisture and appear to produce or be associated with biofilms that form on concrete surfaces.

Appropriate nutrient requirements vary according to the type of MID microorganism. The autotrophic mineral-acid-producing sulfur oxidizing bacteria and nitrifying bacteria require suitable forms of reduced sulfur or nitrogen, respectively, as sources of energy. The heterotrophic organic-acid-producing organisms require a suitable organic substrate such as glucose, starch, glycogen, cellulose, or the like.

Preferably, the MID microorganism utilized in accordance with the method of the present invention is an acid-producing bacterium or fungus, such as belongs to the group consisting of acid-producing heterotrophic bacteria and fungi, nitrifying bacteria, sulfur oxidizing bacteria, and mixtures thereof.

Nitrifying bacteria that can be used advantageously within the scope of the present invention include, but are not limited to, Nitrosomonas and Nitrobacter spp., such as Nitrosomonas europea, Nitrobacter winogradskyi, and Nitrobacter vulgaris. These nitrifying bacteria produce nitric acid that degrades cementitious surfaces.

Sulfur oxidizing bacteria that can be used advantageously within the scope of the present invention include, but are not limited to, Thiobacillus species such as Thiobacillus thiooxidans, Thiobacillus ferrooxidans, Thiobacillus neapolitanus, and Thiobacillus intermedius. These sulfur oxidizing bacteria produce sulfuric acid that degrades cementitiou surfaces.

Organic-acid-producing heterotrophic bacteria and fungi that can be used advantageously within the scope of the present invention include, but are not limited to, numerous bacteria and fungi that produce acetic, citric, formic, lactic, and other carboxylic acids. Illustrative examples of such bacteria and fungi include bacteria from the genus Pseudomonas, such as P. cepacia, and fungi from the genera Aspergillus, such as A. niger and A. phenicis, Penicillium, such as P. herquei, P. funiculosum, P. lanosocerulum, P. similicissum, P. atramentosum, and P. roquefortii, Paecilomyces, Acremonium, Verticillium, Geomyces, and Chrysosporium.

It should be appreciated that other microorganisms capable of microbial influenced degradation are within the scope of the present invention.

The encapsulation material utilized in accordance with the present invention is combined with at least one type of mid microorganism, and includes a supply of appropriate nutrients and moisture. The resulting encapsulated microorganism requires no additional manual application of nutrients or moisture upon application to a contaminated surface for microbial influenced degradation. Thus, the encapsulated microorganism is referred to herein as being passively supplied with moisture and appropriate nutrients.

Suitable encapsulation materials for this invention are natural or synthetic polymeric binder materials. These materials should be able to encapsulate the microorganism, the moisture, and/or nutrients of this present invention in such a way that the nutrients and moisture are available to the microorganism and allow the microorganism to degrade the substrate on which they are applied. Examples of polymeric binder materials are natural and synthetic gels and foams. Natural and/or synthetic gelatin polymers are most preferred.

Preferred encapsulation materials are those which maintain sufficient adhesion to the substrate to adhere to the substrate during the degradation phase such as on a vertical surface, but which are readily removed at the end of the phase. The encapsulation materials preferably can be applied through spraying equipment, are non-toxic to the microorganisms, are gas permeable, hold their characteristics under acidic conditions, are easily applied and removed, and when dealing with radioactive contamination, are resistant to radioactivity.

Suitable examples of gels for encapsulation purposes include, but are not limited to, PrimaCel™, also known as EX-7948, Cellulon®, KelcoGel®, KELGIN®, KELZAN®, all by Monsanto Kelco Industrial Biopolymers. It should be appreciated that other gel-encapsulation materials which provide nutrients, moisture, or both, to the MID microorganisms, such as but not limited to foams or other viscous material, are within the scope of the present invention.

While conventional gels are formed at about 55° C., the preferred gel-encapsulation material, such as that used to encapsulate T. thiooxidans bacteria, is formed at a temperature of less than about 30° C. This relatively low temperature is used to avoid killing the bacteria.

A preferred gel-encapsulation material in accordance with the present invention comprises about 2 to about 5 of dry gel additive blended with about 500 to about 1000 ml of water, about 5 to about 30 grams of sulfur, and about 0.001 to about 0.1 grams of ammonium molybdo-phosphate. Alternatively, semi-dry gel additive is used in place of the dry additive. The mixture is blended in a high-shear mixer to produce a smooth, consistent gel, and then the microorganism, such as T. thiooxidans, is added to the gel by way of a low-shear mixer. A preferred amount of microorganism is about 1×10⁹ to about 1×10¹¹ per liter of the gel. The encapsulated microorganism is then ready to be utilized on a substrate.

A preferred substrate in accordance with the present invention is a contaminated cementitious surface. By definition, cementitious surfaces encompass surfaces having the characteristics of cement. A preferred cementitious surface suitable for the method of the present invention is concrete, examples of which occur in hot cells, ponds, canals, sumps, biological shields, and the like. It should be appreciated that the present invention is in no way limited to use with these specific cementitious surfaces or with cementitious surfaces in general.

Further, by way of example only and not limitation, the preferred cementitious surface for decontamination purposes is contaminated with toxic wastes, environmentally hazardous materials, radioactive materials, and/or other contaminants. Other contaminated surfaces degraded by microbial influenced degradation are also within the scope of the present invention.

To perform the method of the present invention, an effective amount of the encapsulated microorganism such as described above, is sprayed or painted in a substantially uniform coating on a contaminated surface. An effective amount means an amount of a encapsulated microorganism that is sufficient to provide a selected degradative effect and performance under selected parameters. A person of ordinary skill in the art should be able determine such an effective amount without undue experimentation according to the guidelines provided herein. One example of an effective amount of microorganism encapsulation-mixture is a sprayed thickness of about 1 mm to about 5 mm on a substrate.

In the case of a particularly “hot” radioactive area, the gel-encapsulated microorganism is preferably sprayed by means of remote application, which does not require the presence of an actual worker dressed in biological protective wear, and thus reduces exposure and risk.

The encapsulated microorganisms are preferably allowed to remain on the cementitious surface for an amount of time sufficient for microbial influenced degradation activity to degrade the cementitious surface to a selected depth. Those of skill in the art recognize that this sufficient amount of time will vary according to substrate and the depth required to eliminate the contaminated portions of the substrate. For example, microbial influenced degradation of a cementitious surface should extend up to about 4 mm, and preferably up to about 2 mm, into the surface to substantially degrade the contaminated layer. It has been demonstrated that a sufficient amount of time for this selected depth of degradation is about 12-18 months. It follows that an amount of time sufficient for microbial influenced degradation of less depth may take less time.

In a preferred embodiment of the present invention, the adaptability of a commercially available gel material, EX-7948, for the production of a hydrophilic gel matrix containing sulfur oxidizing MID microorganisms, sulfur, and water was demonstrated. The findings illustrate that this invention effectively maintains MID activity under passive conditions, that is, without manual addition of sulfur or water, for an extended period of time.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 

We claim:
 1. A biomediated process for removing contamination from a surface of a substrate, comprising: applying to a surface of a substrate an effective amount of a microorganism capable of microbial influenced degradation activity of said surface of said substrate, wherein the microorganism is encapsulated with a sufficient amount of at least one appropriate nutrient to effect degradation of said substrate to at least a selected depth; and maintaining the encapsulated microorganism on said surface for a time sufficient for microbial influenced degradation activity to degrade said substrate to at least a selected depth.
 2. The process according to claim 1, wherein applying an effective amount of an encapsulated microorganism comprises applying an effective amount of an encapsulated form of a microorganism selected from the group consisting of acid-producing bacteria and fungi.
 3. The process according to claim 2, wherein applying an effective amount of an encapsulated form of a microorganism comprises applying an effective amount of an encapsulated form of an acid-producing bacterium selected from the group consisting of an organic-acid-producing heterotrophic bacteria, nitrifying bacteria, sulfur oxidizing bacteria, and mixtures thereof.
 4. The process according to claim 3, wherein applying an effective amount of an encapsulated form of an organic-acid-producing heterotrophic bacteria comprises applying an effective amount of an encapsulated form of an organic-acid producing heterotrophic bacteria that produces an organic acid selected from the group consisting of acetic, citric, formic, and lactic acids.
 5. The process according to claim 3, wherein applying an effective amount of an encapsulated form of a nitrifying bacteria comprises applying an effective amount of an encapsulated form of a nitrifying bacteria selected form the group consisting of Nitrosomonas, Nitrobacter, and mixtures thereof.
 6. The process according to claim 3, wherein applying an effective amount of an encapsulated form of a sulfur oxidizing bacteria comprises applying an effective amount of an encapsulated form of a sulfur oxidizing bacteria selected from the group consisting of Thiobacillus thiooxidans, Thiobacillus ferrooxidans, Thiobacillus neapolitanus, Thiobacillus intermedius, and mixtures thereof.
 7. The process according to claim 2, wherein applying an effective amount of an encapsulated form of a microorganism comprises applying an effective amount of an encapsulated form of an organic-acid-producing heterotrophic fungus.
 8. The process according to claim 7, wherein applying an effective amount of an encapsulated form of an organic-acid-producing heterotrophic fungus comprises applying an effective amount of an encapsulated form of an organic-acid-producing heterotrophic fungus selected from the group consisting of Aspergillus, Penicillium, Paecilomyces, Acremonium, Verticillium, Geomyces, Chrysosporium, and mixtures thereof.
 9. The process according to claim 1, wherein applying an effective amount of an encapsulated form of an encapsulated microorganism comprises applying an effective amount of an encapsulated microorganism that is passively supplied with moisture.
 10. The process according to claim 1, further comprising providing an encapsulated microorganism for applying to said surface by combining a microorganism with an encapsulation material.
 11. The process according to claim 10, wherein combining a microorganism with an encapsulation material comprises combining said microorganism with a polymeric binder material.
 12. The process according to claim 11, wherein combining said microorganism with a polymeric binder material comprises combining said microorganism with a gel, foam, or other viscous material.
 13. The process according to claim 1, wherein applying an effective amount of an encapsulated microorganism capable of microbial influenced degradation activity on said surface of a substrate comprises applying an effective amount of said encapsulated microorganism to a surface of a cementitious material.
 14. The process according to claim 13, wherein applying an effective amount of said encapsulated microorganism to a surface of a cementitious material comprises applying an effective amount of said encapsulated microorganism to concrete.
 15. The process according to claim 14, wherein applying an effective amount of said encapsulated microorganism to concrete comprises applying an effective amount of said encapsulated microorganism to a hot cell, pond, canal, sump, biological shield, room, floor, wall, or storage tank.
 16. The process according to claim 1, wherein applying an effective amount of an encapsulated microorganism capable of microbial influenced degradation activity on said surface of a substrate comprises applying an effective amount of said encapsulated microorganism to a surface contaminated by a radionuclide.
 17. The process according to claim 1, wherein maintaining the encapsulated microorganism on said surface for a time sufficient for microbial influenced degradation activity to degrade said substrate to a selected depth comprises maintaining the encapsulated microorganism on said surface for a time sufficient for microbial influenced degradation activity to degrade said surface to a depth of up to about 4 mm.
 18. The process according to claim 1, wherein maintaining the encapsulated microorganism on said surface for a time sufficient for microbial influenced degradation activity to degrade said substrate produces a degraded layer.
 19. The process according to claim 18, further comprising removing said degraded layer for disposal. 